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Video: Two Galaxies on a Collision Course

How do massive galaxies form? A popular hypothesis is that they start out small and grow over time by swallowing other smaller galaxies. But new observations by Herschel, a far infrared space observatory operated by the European Space Agency, show that massive elliptical galaxies can form from the merger of two large galaxies. Herschel spotted two large galaxies—11 billion light-years away—in close proximity to one another, both of them making new stars at a much higher rate than most galaxies from that cosmic period. Follow up observations with other telescopes confirmed that the two galaxies were in the act of merging (see video). Each is making new stars on the order of 2000 suns every year. In about 200 million years, the galaxies will have completed their merger and settled down to a much lower star forming rate, the team reports online today in Nature. Two big beasts, subsumed by one another, will have become one monster of a galaxy.